The majority of data reported in open literature on hydride embrittlement of Zr alloys is based on hydrogen not deuterium which evolves during aqueous corrosion of in-core components in pressurized heavy water reactor operations. It is assumed that hydrogen and deuterium have similar effects on tensile properties and fracture toughness of Zr alloys. The thermal and physical properties as well as stress free transformation strain of hydrides and deuterides are different. Because of these differences, hydrogen and deuterium may affect tensile and fracture properties of Zr alloys to a different extent. The hydrogen isotope effects on tensile and fracture properties were investigated using samples machined from Zr–2.5Nb pressure tube (PT) material charged with hydrogen between 30 and 200 wppm and tests were carried out between 25 and 300 °C. The tensile properties and fracture toughness of the hydrogen charged Zr–2.5Nb PT material were compared to the Zr–2.5Nb PT material charged with deuterium. The observed effect of hydrogen isotopes on tensile and fracture toughness was rationalized in terms of stress free transformation strain and the temperature dependence of the fracture toughness of the hydrided material was correlated with the presence of axial splits on fracture surfaces.